All Free Biology MCQs with Answers

1561. Chlorosis, i.e., yellowish hue on the leaves results from_____________?

• A. Accumulation of toxic waste products in leaves.
• B. Deficiency of chlorophyll.
• C. Short supplies of mineral nutrients in the soil.
• D. All of these

Explanation: Chlorosis is the yellowing of leaves caused by reduced chlorophyll, and it can result from mineral deficiency or injury caused by accumulated toxic substances. Students often choose only mineral deficiency because it is a common cause, but the wording asks for all listed causes, making all of these appropriate.

1562. The one which is a weed killer is___________?

• A. IAA
• B. NAA
• C. 2 4-D
• D. GA

Explanation: 2,4-D is a synthetic auxin that acts as a selective herbicide, especially against many broad-leaved weeds. IAA and NAA are auxins mainly discussed as natural or synthetic plant-growth regulators, while GA is gibberellic acid and promotes growth. Students may choose IAA because it is an auxin, but it is not the standard weed killer named here.

1563. The one which causes contraction of wall of the uterus during and after birth?

• A. ADH
• B. MSH
• C. Oxytocin
• D. Progesterone

Explanation: Oxytocin stimulates contraction of the uterine smooth muscle during labour and also helps the uterus contract after birth, reducing bleeding. ADH mainly promotes water reabsorption, while progesterone supports pregnancy rather than directly causing labour contractions. Students may choose progesterone because it is a reproductive hormone, but oxytocin is the hormone responsible for uterine contraction.

1564. Exposure to low temperature stimulates plants to flower. This is called___________?

• A. Photoperiodism
• B. Thermotropism
• C. Vernalisation
• D. Thermoregulation

Explanation: Vernalisation is the induction or acceleration of flowering after exposure to a prolonged period of low temperature. The likely mistake is photoperiodism, which concerns flowering responses to day length rather than cold treatment.

1565. Sleep movements are a type of___________?

• A. Turgor movements
• B. Growth movements
• C. Tactic movements
• D. Paratonic movments

Explanation: Sleep movements in plants are usually turgor movements, produced by reversible changes in water content and pressure in specialised motor cells, often at the leaf base. The likely mistake is growth movements, which are irreversible because they result from unequal growth rather than temporary changes in turgor.

1566. In humans no of tetrads formed during mitosis is __________?

• A. 23
• B. 46
• C. 0
• D. None of These

Explanation: Tetrads form in prophase I of meiosis when homologous chromosomes pair and undergo crossing over. Mitosis does not pair homologous chromosomes, so the number of tetrads formed during human mitosis is 0. Students may choose 23 by confusing tetrads with the 23 pairs of human chromosomes.

1567. Tjio and Levan's contribution is very significant because they __________ ?

• A. Identified barr bodies
• B. Detected sex linkage
• C. Pointed out mutational changes
• D. Gave the number of human chromosomes

Explanation: Tjio and Levan established in 1956 that the normal human diploid chromosome number is 46, correcting the earlier accepted count of 48. Barr bodies, sex linkage, and mutation studies were associated with other discoveries. Students may choose identification of Barr bodies because both topics concern chromosomes, but Barr bodies were described by Murray Barr and Ewart Bertram.

1568. A cistron is ________?

• A. Structural of gene
• B. Functional unit of RNA
• C. Functional unit gene
• D. Replication unit of gene

Explanation: A cistron is the functional unit of a gene, specifically a DNA segment that codes for one polypeptide or functional RNA product. It is not merely the structural part of a gene, and replication is associated with a replicon rather than a cistron. Students may choose the replication unit because both terms describe gene-related DNA regions, but their functions differ.

1569. Balbiani discovered special type of chromosome from the salivary gland of chironomus larva which are recognized by the presence of ________?

• A. Bands
• B. Loops
• C. Both bands and loops
• D. All of the above

Explanation: Balbiani's giant polytene chromosomes in Chironomus larvae show a characteristic pattern of dark transverse bands and expanded regions or loops called puffs. These structures result from repeated DNA replication without cell division and active transcription at the puffs. Students may choose only bands because they are the most visible feature, but the special chromosomes are recognized by both bands and loops or puffs.

1570. How many nucleosomes are found in helical coil of 30 nm chromatin fibre ____?

• A. 10
• B. 12
• C. 06
• D. 09

Explanation: In the traditional solenoid model of the 30 nm chromatin fibre, six nucleosomes are arranged in one helical turn. Therefore the expected number is 6 nucleosomes, written as 06 in option c. Students may choose 10 or 12 by confusing the 30 nm fibre with another level of chromatin packing or with a different model.